Oil-Water Separators Rise to the Top

Technically speaking focuses on how oil-water separators work and where they are required. Two University of Wisconsin experts give us and exhaustive assessment.

In the past, some operators disposed of oily wastewater in earthen pits, septic tanks or injection well systems. Today, such practices may be followed only under close restrictions by the U.S. EPA (under its broad effort to protect and clean up groundwater) and can lead to costly cleanups. So the smart thing is to find an alternative way to deal with this problem. An oil and water separator (OWS) system might be the answer. Here are some things you may need to know about OWS systems.

Know the rules of runoff
The 1972 amendments to the Federal Water Pollution Control Act (also referred to as the Clean Water Act) prohibits the discharge of any pollutant to waters of the United States from a point source unless authorized by a National Pollutant Discharge Elimination System (NPDES) permit. Some PE&T readers with oil-contaminated wastewater have avoided the NPDES permit hassle by discharging wastewater to sanitary sewers; however storm water of any kind may not be discharged into sanitary sewers serving publicly owned treatment works (POTW).

Most automotive refueling and service facilities come under NPDES provisions that are applicable to “commercial, retail, light industrial and institutional facilities.” Applications for NPDES stormwater permits for such facilities must be submitted by September 1, 2001. Generally, the upper limit on “fat-oil-grease” (FOG) concentrations in stormwater discharges under NPDES permits is 100 ppm. However, most local ordinances limit the FOG concentrations of wastewater to 10 ppm.

To be sure you know and understand the requirements applicable to your operation, check with local EPA, ecology, or other authorities having jurisdiction (AHJ). In addition to the current requirements, they should also tell you about any anticipated changes that you need to consider.

The proof is in the testing
You need to know whether the runoff of storm water or wastewater from your facility contains fat, oil or grease and, if so, whether this runoff exceeds the effluent limit of 10 ppm FOG (or whatever the limit is in your location). Beware. Here is where looks can deceive. For example, the commonly observed bluish, luminous film or sheen, typical of hydrocarbon oil or grease on water, may result from an extremely thin “monomolecular” surface film. And, while the presence of FOG is obvious, the actual concentration in a large volume of water flow may be negligible.   

Only by competent laboratory testing will you know the FOG concentration level for certain. Testing is also necessary to determine the contaminants’ specific gravity and their separability from the water. If tests show that your FOG level is negligible, you may still choose to install a skimmer for esthetic reasons or just to be a good citizen. But if the level exceeds the limit, you must do something about it. And if the FOG content is free to separate, an OWS could be the answer. Your trade association should be able to advise you on the availability of qualified consultants and laboratories for the tests you will need.

Separating the O from the W
It is a given that oil and water will separate (the oil will rise to the top) if allowed to sit still or nearly still for a sufficient amount of time, in an appropriate area. Gravity OWS systems provide optimum conditions under which this separation can occur, thus providing a reliable first step toward cleaning up the effluent before it is discharged into the environment. Once separated, the FOG contaminants can be removed and properly disposed of or recycled.

Several conditions can interfere with or preclude oil and water separation. These include even slight turbulence due to flow; the formation of solutions or stable emulsions by detergents or other chemicals; or, as is most frequently the case, the formation of sludge by the settling of grit and other solids. Only “free” oil will separate in an OWS—dissolved materials or stable emulsions cannot be separated or removed in this manner. Dissolved materials or emulsions can include the BTEX compounds, fuel additives, gasoline and soluble petroleum products.

The drawing (below) illustrates the principal components of an industrial OWS, including the inlet devices to produce a quiescent flow condition and the provision for “skimming” to recover the oil or grease. A coalescing unit greatly increases the capacity and efficacy of an OWS. Predesigned and packaged units take many external shapes, but should include each of these features.

This FRP OWS system provides coalescing plates and modular construction. The OWS can remove free oil droplets equal to and greater than 20 microns. Courtesy of Fluid Containment

Go with the flow
To select an OWS that functions properly, it is important to know the flow rate, flow conditions and contaminant properties at your site. The size of most OWS systems is based on flow rates. The flow rate, along with the specific gravity of the material, determines how much retention time and coalescing material the OWS must have to be effective. The lighter or the lower the specific gravity of the oil, the easier to separate it from the water. Thus, the OWS may operate at a higher flow rate.

It makes sense to know, or to have good estimates of, the maximum (or design) wastewater flow rate and the lowest temperature corresponding to the design flow. This is because temperature affects specific gravity and viscosity of the liquids to be separated. Cold weather will be the limiting condition for an OWS.

It also makes sense to avoid any pumping of the wastewater before it enters an OWS. Pumping would tend to emulsify the liquid mixture and thereby reduce the OWS’ effectiveness.

OWS O&M
The operation and maintenance of an OWS system is simple but very critical to its effectiveness. The system’s performance should be monitored routinely. Timely housekeeping chores are necessary to remove or keep out foreign objects such as tools, gloves and rags; grit or sludge buildup on surfaces or in the bottom of the OWS or grit chamber (if present); floating solids or “slimes” that foul the skimmer or plug the drain or pumping lines; or any of the above that may foul the tube or plate bundle (if present). If these basic chores are performed regularly, an OWS should provide excellent service for a long time.

Repetition good for the soil
The essential things to know and consider in deciding on whether to install an OWS, and on its size and type are:

• The FOG content of the storm water or wastewater at your site compared to the NPDES limitation.
• The specific gravity of the FOG materials and their separability from the water.
• The flow rate and flow conditions at the site.

While getting consultation and advice on these matters from competent sources may seem expensive, the cost of correcting poorly designed and installed systems for handling effluent can be much greater. A soundly designed OWS system, if properly monitored and maintained, can provide trouble free service for many years.

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